1. Field of the Invention
The present invention relates to optical communications systems and, in particular, to a wide band optical amplifier.
2. Description of the Related Art
In recent years, an erbium-doped fiber amplifier (EDFA) applicable to an optical communication system has been put to practical use. For example, a plurality of optical light signals each having a different wavelength is multiplexed together into a wavelength division multiplexed (WDM) signal light. The broad band of the optical amplifier enables each signal light in the wavelength division multiplexed (WDM) signal light to be amplified. In systems employing EDFAs, channel density has been limited by the usable gain bandwidth of the EDFA. Thus, increasing the EDFA gain bandwidth increases the system capacity while maintaining channel spacing and bit rate per channel.
Many different techniques have been used to achieve a wide gain in the conventional wavelength range between 1530 nm-1560 nm (hereinafter referred to as xe2x80x9cC-bandxe2x80x9d) and long wavelength range between 1570 nm-1610 nm (hereinafter referred to as xe2x80x9cL-bandxe2x80x9d) for the wavelength-division-multiplexed (WDM) transmission system. FIG. 1 illustrates a schematic view of a broadband EDFA according to a prior art system. The input band signals pass through two amplification stages, the C-band and the L-band, then recombined afterward to produce broad optical bandwidth. Typically, the L-band fiber amplifier requires a lengthy EDF (188 m) as the power conversion efficiency is lower than the conventional C-band EDFAs. In this type of amplifying medium, forward amplified spontaneous emission (hereinafter referred to as xe2x80x9cASExe2x80x9d) light propagating in the same direction as the propagation direction of signal light and backward ASE light propagating in the direction opposite to the propagation direction of signal light are generated. In the case that a number of repeaters, each with an optical amplifier, is provided to an optical fiber transmission line, the noise spectrum ASE generated in the optical amplifier causes problem of precisely monitoring the optical power relating to the signal spectrum. Accordingly, there is a need for a cost-effective optical power amplifier with improved power conversion efficiency and reduced noise.
The present invention is directed to an optical amplifier with a structure that efficiently utilizes erbium-doped fiber amplifiers (EDFAs) and the associated pump source, and as a consequence provides an increased signal bandwidth with less noise level by utilizing shorter erbium-doped fibers (EDFs) and less pump consumption.
The present invention relates to an optical amplifier for amplifying optical signals encompassing two or more optical bands. The amplifier includes a first erbium-doped fiber amplification pumped by a first pump light source; a second erbium-doped fiber amplifier pumped by a second pump light source; a split section disposed between the first amplifier and the second amplifier stages for splitting optical signals received thereon into a number of sub-band signals; a reflector for reflecting the output of the second amplifier stage back into the second amplifier stage in a reverse direction; a combiner for recombining the amplified light signals to produce an output signal; and, a circulator, provided at the output of the split section, for redirecting the reversely amplified output from the second amplifier stage to the input of the combiner and for preventing ASE generated from the second amplifier stage to travel to the input of the combiner.
The present invention relates to a method of amplifying optical signals which is performed according to the following steps: passing input optical signals through a first amplifier stage for amplification; splitting the amplified input signals into C-band and L-band signals; amplifying the L-band signal further in a second amplifier stage; redirecting the amplified L-band again back into the second amplifier stage in a reverse direction; and, recombining the further amplified L-band and the C-band to produce an output signal.
The foregoing and other features and advantages of the invention will be apparent from the following, more detailed description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, the emphasis instead is placed upon illustrating the principles of the invention.